1. Field of the Invention
The present invention relates to methods of recovering methane from solid carbonaceous subterranean formations and, more particularly, to methods of increasing the recovery rate of the methane.
2. Setting of the Invention
In the recovery of methane from subterranean formations of solid carbonaceous material, a rapid recovery of methane is desired so that income from the sale of methane can be increased. Currently, the rate of methane recovery in most projects is dependent on how rapidly the formation pressure adjacent production wells can be decreased One method that does not depend upon formation pressure decrease to recover methane involves the injection of a gas into the subterranean formation to cause methane to be desorbed by displacement or stripping. Examples of this method are disclosed in Puri et al., U.S. Pat. No. 4,883,122. This method has the potential to significantly increase the overall volume of methane recovered in the project, as well as increase the rate of methane recovery over methods that depend upon formation pressure decrease.
The rate of methane recovery is directly related to the rate of desorbing gas injection, so to increase the rate of recovery, a project operator desires to increase the gas injection rate. However, the gas injection rate is controlled by the near wellbore permeability, which in turn is influenced by the gas content within water in the cleat system of the subterranean formation. For example, the injected gas cannot flow with ease into the solid carbonaceous subterranean formation until the cleat system is dewatered or until the cleat system has changed from being water saturated to gas saturated. This flow restriction is referred to as a relative permeability effect. Increasing the injection rate to overcome the relative permeability effect is restricted because a higher injection rate results in a higher wellbore bottomhole pressure. If the wellbore bottomhole pressure exceeds the fracture pressure of the solid carbonaceous subterranean formation then the injected gas will flow relatively quickly and preferentially into the resulting fractures and pass out into the subterranean formation without adequately contacting the near wellbore area to change the area to being gas saturated.